1. Field
The present disclosure relates to a press molding apparatus and a press molding method.
2. Description of the Related Art
In recent years, in order to reduce the weight of vehicles, a stampable sheet (fiber reinforced composite) obtained by combining thermoplastic resin and reinforcing fiber, such as glass fiber or carbon fiber, has been used as a material of component parts. The stampable sheet is a base material obtained by impregnating reinforcing fiber with thermoplastic resin and then forming the composite into a planar shape.
An example of a processing method for a stampable sheet is a press molding process in which a heated and softened stampable sheet is pressed by a pair of dies. In this molding method, a stampable sheet disposed in a cavity defined by a lower die and an upper die is pressed so that the stampable sheet flows in and fills the cavity, for example, a pressed portion escapes or a pressed portion pulls another portion, so as to provide a molded product that has sheet thickness and shape as desired.
Note that when the stampable sheet is heated, the resin restraining the reinforcing fiber melts and the reinforcing fiber is released from the restraint, causing a spring back. Therefore, the sprung-backed material needs to be pressed to an original thickness in order to recover the physical property (mechanical characteristic) of the material.
However, even though the material is pressed to the original sheet thickness and assumes the thickness determined by the dies, the flowage of the stampable sheet in the cavity during the press molding process results in an insufficient weight per unit area (basis weight) and spring back of the material remains (i.e., void remains in the material and the density thereof remains low), giving rise to a risk of reducing the connection between the reinforced resin and the fiber or the like and therefore causing deterioration of the physical property (mechanical characteristics) of the molded product.
Furthermore, a variation in basis weight of the stampable sheet prior to the molding process, which is another cause for an insufficient basis weight of the molded product, also gives rise to a problem of the molded product similar to the aforementioned problem.
In view of such circumstances, Japanese Unexamined Patent Application Publication No. 9-267344 proposes that a stampable sheet that, before being heated, has a greater thickness than the molded product has be used so that the volume of the thermoplastic resin will not become insufficient at various sites in the cavity.
Furthermore, in Japanese Unexamined Patent Application Publication No. 9-267344, in order to cause surplus thermoplastic resin to flow out of the die through an edge portion side of the cavity, an end edge of an opening portion of the die is provided with a cutout (overflow gate).
However, in some cases, a die has, at a site at an inward side of the edge portion of the cavity, protuberances and depressions for giving a shape to the molded product. When a stampable sheet is pressed by such dies, there is possibility that the material may flow as if it is drawn in by protuberances and depressions and part of the material may reside in large amounts at the inward side of the opening portions of the dies.
In other words, due to the presence of protuberances and depressions or due to large flow resistance of the material depending on the area, there is a risk of surplus thermoplastic resin (waste resin) failing to reach an opening portion of a die which is an ejection portion provided at a terminal end. Then, the surplus thermoplastic resin does not flow out of the dies through the overflow gate but excess amounts of thermoplastic resin (hereinafter, referred to as “excess portions”) reside at sites near protuberances and depressions.
As a result, a hard stop happens in which the internal die pressure (reaction force) from the excess portions exceeds the pressing force of the press machine before closure of the pair dies and therefore the dies cannot be tightly closed.
Possibility of appearance of an excess portion exists not only in the foregoing case but also in the case where the stampable sheet has a basis weight variation and the like.